drm/nouveau/flcn/qmgr: allow arbtrary priv + return code for callbacks

Code to interface with LS firmwares is being moved to the subdevs where it
belongs, rather than living in the common falcon code.

Arbitrary private data passed to callbacks is to allow for something other
than struct nvkm_msgqueue to be passed into the callback (like the pointer
to the subdev itself, for example), and the return code will be used where
we'd like to detect failure from synchronous messages.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>